I've installed the lastest build ( 5.9.0.13) recently and noticed that the restores I/O is twice as low as the original version ( 5.2.1.7 ). This is impacting our servers as we have some somewhat large DBs. Nightly development restores, logshipping etc has slowed to the point of being useless. What can I do? Is anyone else experiencing this?

This forum post has raised a support request in our support system. I am shortly responding to this with a few quick questions to address your issues. We can leave this post open for other current users to post their comments and feedback.

Same experience after upgrade from 4.0.31 to 5.9.0.13. Average backup time has increases by a factor of 2.5.
We have not changed the configuration option:
Not using zip compatible or fast compression.
Not using encryption
Dynamic file updates disabled and not formating as virtual restore source or Oracle backup

As I mentioned, our configuration has not changed between the two versions (4.0.31 to 5.9.0.13). That is at least if one can assume that the meainig of UI fields has not changed. We also don't have the cycles to spend on testing earlier 5.x releases.
We will remain on 4.0.31 until we migrate to SQL Server native compression to reduce our configuration footprint in the year ahead.